The schools that manage their own admissions (eg voluntary aided, foundation, academy and free schools) set their own rules and then manage their own admissions. A few of them follow Hertfordshire County Council's rules for community and voluntary controlled schools, while others have rules that are unique to their school.
Once you've selected a school, using the Admission Calculator, the admission rules will pop up under the map. Go through the rules in turn, starting at 1, then 2, then 3, and so on, and find the first one that applies to you. Put this rule in the Calculator, as it's the rule you will be considered under if the school is oversubscribed. At the top of the rules, you can see what type of school it is (eg academy mixed-sex primary school).